This study addresses the counselling of heterosexual men with low-risk behaviour who, voluntarily or involuntarily, take an HIV test.

If such a man tests positive, the chance that he is infected can be as low as 50%. We study what information counsellors communicate to clients concerning the meaning of a positive test and whether they communicate this information in a way the client can understand.

To get realistic data, one of us visited as a client 20 public health centres in Germany to take 20 counselling sessions and HIV tests.

A majority of the counsellors explained that false positives do not occur, and half of the counsellors told the client that if he tests positive, it is 100% certain that he is infected with the virus.

Counsellors communicated numerical information in terms of probabilities rather than absolute frequencies, became confused, and were inconsistent.

Based on experimental evidence, we propose a simple method that counsellors can learn to communicate risks in a more effective way.